1. Field of the Invention
The present invention relates to a novel process for producing a positive electrode active material for a lithium secondary cell.
2. Discussion of Background
Many of positive electrode active materials used in lithium secondary cells which are practically employed at present, are made of lithium cobalt oxide. However, lithium secondary cells are required to have a larger cell capacity, and lithium nickel oxide has been studied as a new positive electrode active material.
However, lithium nickel oxide undergoes crystal expansion or shrinkage upon absorption or desorption of lithium. Especially when at least 80% of lithium is desorbed, irreversible shrinkage of crystal takes place, and it is known that the cell capacity decreases substantially by repetition of absorption and desorption of lithium i.e. repetition of charging and discharging (T. Ohzuku, et.al., J. Electrochem. Soc., 140, 1862, (1993)).
Further, when lithium nickel oxide is used as a positive electrode active material, a phenomenon of abrupt heat generation due to oxidation of an electrolyte in a cell by highly oxidized nickel oxide, is observed during charging. This is a serious problem against practical use of lithium nickel oxide. To solve this problem, it has been proposed to replace a part of nickel with aluminum (T. Ohzuku, et. al., J. Electrochem. Soc., 142, 4033, (1995)). However, with the composition shown by this study, the cell capacity per unit mass is not substantially different from one where lithium cobalt oxide is used, and the merit for high cell capacity by the use of lithium nickel oxide tends to be small.
As a method for solving such problems, JP-B-2770154 and JP-A-8-222220 disclose a case wherein an improvement has been attempted by replacing a part of nickel with cobalt.
Such a positive electrode active material of the formula LipCoqNi1xe2x88x92qO2 wherein 0.95 less than p less than 1.05, and 0.15 less than q less than 0.25, having a part of nickel substituted with cobalt, is capable of solving the problems such as cycle deterioration and heat generation when the conventional lithium nickel oxide is used as a positive electrode active material, while maintaining a high cell capacity which is a characteristic of lithium nickel oxide. However, in its production, no adequate high cell capacity has been obtained by a method wherein a lithium salt, a cobalt salt and a nickel salt are simply mixed and fired, and a method of using a coprecipitated salt of cobalt and nickel has been attempted. A synthetic method for such a coprecipitated salt may, for example, be a method wherein cobalt chloride and nickel chloride are dissolved in pure water having carbon dioxide gas saturated therein, and sodium bicarbonate is added thereto for coprecipitation to obtain a basic carbonate (JP-B-2770154) or a method wherein an alkali solution is added to an aqueous solution of cobalt sulfate and nickel sulfate to obtain a coprecipitated hydroxide (JP-A-8-222220). In either case, an alkali metal such as sodium remains in the precipitates, and such an alkali metal remains in the positive electrode active material, whereby it has been impossible to minimize deterioration by repetition of charging and discharging. Further, in a case where a basic carbonate coprecipitated with sodium bicarbonate, is used, there has been a drawback that the heat generation initiation temperature during charging is low.
It is an object of the present invention to provide a process for producing a lithium-containing complex oxide containing nickel and cobalt together with lithium, whereby the cycle deterioration and heat generation behavior during charging can be overcome while maintaining a high cell capacity.
That is, the present invention provides a process for producing a positive electrode active material for a lithium secondary cell comprising a lithium-containing complex oxide of the formula LixCoyNi1xe2x88x92yO2 wherein 0.95xe2x89xa6xxe2x89xa61.05, and 0.05xe2x89xa6yxe2x89xa60.50, which comprises heating an aqueous solution containing an ammine cobalt salt and an ammine nickel salt to form a salt containing cobalt and nickel, then mixing the salt with a lithium compound and firing the obtained mixture at a temperature of from 600 to 850xc2x0 C.
In the present invention, x in the formula for the lithium-containing complex oxide is from 0.95 to 1.05, preferably from 0.95 to 1.00. Further, y is from 0.05 to 0.50, preferably from 0.1 to 0.3, particularly preferably from 0.15 to 0.25, from the cell capacity and the high temperature safety when the positive electrode active material is used for a lithium secondary cell.
The lithium-containing complex oxide obtained by the process of the present invention is excellent in the high cell capacity and the heat generation behavior during charging. Further, a salt containing cobalt and nickel (hereinafter referred to also as a cobalt nickel salt) is formed without adding an alkali metal compound to an aqueous solution of a mixture of a cobalt salt and a nickel salt, whereby cobalt and nickel are uniformly distributed with little impurities of alkali metals such as sodium.
A lithium compound is added to the cobalt nickel salt thereby obtained, followed by firing, whereby it is possible to obtain a lithium cobalt nickel oxide having cobalt and nickel uniformly solid-solubirized, with little impurities of alkali metals. An alkali metal such as sodium is believed to enter into lithium sites in the lithium cobalt nickel oxide to form a defect in the crystal and thereby to hinder transfer of lithium, which in turn causes deterioration of the charging and discharging capacity. Accordingly, it is extremely useful to obtain a positive electrode active material of such a lithium-containing complex oxide with little impurities.
The valences of cobalt and nickel in the ammine cobalt salt and the ammine nickel salt to be used in the present invention, are riot particularly limited. However, it is preferred that cobalt is bivalent or a mixture of bivalent and trivalent, and nickel is bivalent. The ammine cobalt salt and the ammine nickel salt to be used in the present invention, are preferably carbonates, sulfates or nitrates. It is particularly preferred that either, particularly each, of them is a carbonate, since the solubility in water is high, and it is easy to form a complex salt of cobalt and nickel having high uniformity by heating. By heating an aqueous solution containing an ammine cobalt carbonate and ammine nickel carbonate, it is possible to obtain a salt (a basic carbonate) of cobalt and nickel.
The temperature for heating the aqueous solution containing an ammine cobalt salt and an ammine nickel salt is preferably from 100 to 150xc2x0 C., particularly preferably from 120 to 140xc2x0 C., since the thermal decomposition of an ammine cobalt salt and an ammine nickel salt usually takes place at a temperature of at least 100xc2x0 C., although it depends also on the types of the salts.
Further, during the reaction, ammonia gas or the like will be generated. For example, in the case of a carbonate, carbon dioxide gas and ammonia gas will be generated. The pressure in the reaction system at that time is preferably within a range of from atmospheric pressure to 0.5 Mpa.
Thus, a salt is formed by permitting thermal decomposition to take place at a high temperature, whereby a salt is obtainable wherein cobalt and nickel are uniformly distributed. Consequently, in the obtained positive electrode active material, there will be no irregularity in the composition due to segregation of cobalt or nickel. Accordingly, it is believed that in a lithium secondary cell employing the positive electrode active material obtained by the present invention, the heat generation behavior during charging is improved.
Then, the salt obtained as described above, and a lithium compound are mixed, and this mixture is fired to obtain a desired lithium cobalt nickel oxide. The lithium compound to be used here, is not particularly limited, but a hydroxide, an oxide or a carbonate is preferred. Especially, lithium hydroxide is preferred from the viewpoint of the charging and discharging cycle durability and the cell capacity of a lithium secondary cell employing the positive electrode active material thereby obtained.
The firing temperature of the above mixture is from 600 to 850xc2x0 C., preferably from 700 to 800xc2x0 C. If it is lower than 600xc2x0 C., the reaction tends to be inadequate, whereby presence of nickel oxide or the like tends to be observed by the X-ray diffraction measurement. If it exceeds 850xc2x0 C., nickel tends to be included in lithium sites, whereby capacity decrease is likely to result, and further, if it becomes at least 900xc2x0 C., lithium tends to be remarkably evaporated, whereby the capacity tends to remarkably decrease. The firing is preferably carried out in two steps as described below, whereby the resulting lithium cobalt nickel oxide will be more homogenized, whereby the heat generation behavior during charging will be improved.
Namely, the mixture of the lithium compound and the salt containing cobalt and nickel, is heated at a temperature of from 300 to 600xc2x0 C., preferably from 450 to 550xc2x0 C., then mixed again, for example, by means of e.g. a mortar or blender, so that the obtained powder is homogenized, and further fired at a temperature of from 600 to 850xc2x0 C., preferably from 700 to 800xc2x0 C. By the firing at a temperature of from 300 to 600xc2x0 C., decomposition of the cobalt nickel salt and the lithium compound is permitted to take place slowly, followed by mixing again, whereby lithium, cobalt and nickel are homogenized in the mixture, whereby the obtained lithium cobalt nickel oxide will be homogenized, and the heat generation behavior during charging will thereby be improved. If the interior of the furnace is reducing by an inert atmosphere such as nitrogen or by a decomposition gas of an organic substance, during the second firing, nickel tends to be reduced to form a nickel (bivalent) oxide phase or nickel metal, and accordingly, it is preferred to maintain the internal atmosphere at an oxygen concentration of at least 25%.
A method for producing an active material for a lithium secondary cell by using a lithium-containing complex oxide obtained as described above, can be carried out by a known method.